Class: Concurrent::Agent

Overview

Agent is inspired by Clojure's agent
function. An agent is a shared, mutable variable providing independent,
uncoordinated, asynchronous change of individual values. Best used when
the value will undergo frequent, complex updates. Suitable when the result
of an update does not need to be known immediately. Agent is (mostly)
functionally equivalent to Clojure's agent, except where the runtime
prevents parity.

Agents are reactive, not autonomous - there is no imperative message loop
and no blocking receive. The state of an Agent should be itself immutable
and the #value of an Agent is always immediately available for reading by
any thread without any messages, i.e. observation does not require
cooperation or coordination.

Agent action dispatches are made using the various #send methods. These
methods always return immediately. At some point later, in another thread,
the following will happen:

The given action will be applied to the state of the Agent and the
args, if any were supplied.

The return value of action will be passed to the validator lambda,
if one has been set on the Agent.

If the validator succeeds or if no validator was given, the return value
of the given action will become the new #value of the Agent. See
#initialize for details.

If any observers were added to the Agent, they will be notified. See
#add_observer for details.

If during the action execution any other dispatches are made (directly
or indirectly), they will be held until after the #value of the Agent
has been changed.

If any exceptions are thrown by an action function, no nested dispatches
will occur, and the exception will be cached in the Agent itself. When an
Agent has errors cached, any subsequent interactions will immediately throw
an exception, until the agent's errors are cleared. Agent errors can be
examined with #error and the agent restarted with #restart.

The actions of all Agents get interleaved amongst threads in a thread pool.
At any point in time, at most one action for each Agent is being executed.
Actions dispatched to an agent from another single agent or thread will
occur in the order they were sent, potentially interleaved with actions
dispatched to the same agent from other sources. The #send method should
be used for actions that are CPU limited, while the #send_off method is
appropriate for actions that may block on IO.

Example

defnext_fibonacci(set=nil)return[0,1]ifset.nil?set+[set[-2..-1].reduce{|sum,x|sum+x}]end# create an agent with an initial value
agent=Concurrent::Agent.new(next_fibonacci)# send a few update requests
5.timesdoagent.send{|set|next_fibonacci(set)}end# wait for them to complete
agent.await# get the current value
agent.value#=> [0, 1, 1, 2, 3, 5, 8]

Observation

Agents support observers through the Observable mixin module.
Notification of observers occurs every time an action dispatch returns and
the new value is successfully validated. Observation will not occur if the
action raises an exception, if validation fails, or when a #restart occurs.

When notified the observer will receive three arguments: time, old_value,
and new_value. The time argument is the time at which the value change
occurred. The old_value is the value of the Agent when the action began
processing. The new_value is the value to which the Agent was set when the
action completed. Note that old_value and new_value may be the same.
This is not an error. It simply means that the action returned the same
value.

Nested Actions

It is possible for an Agent action to post further actions back to itself.
The nested actions will be enqueued normally then processed after the
outer action completes, in the order they were sent, possibly interleaved
with action dispatches from other threads. Nested actions never deadlock
with one another and a failure in a nested action will never affect the
outer action.

Nested actions can be called using the Agent reference from the enclosing
scope or by passing the reference in as a "send" argument. Nested actions
cannot be post using self from within the action block/proc/lambda; self
in this context will not reference the Agent. The preferred method for
dispatching nested actions is to pass the Agent as an argument. This allows
Ruby to more effectively manage the closing scope.

NOTE Never, under any circumstances, call any of the "await" methods
(#await, #await_for, #await_for!, and #wait) from within an action
block/proc/lambda. The call will block the Agent and will always fail.
Calling either #await or #wait (with a timeout of nil) will
hopelessly deadlock the Agent with no possibility of recovery.

Thread-safe Variable Classes

Each of the thread-safe variable classes is designed to solve a different
problem. In general:

Agent: Shared, mutable variable providing independent,
uncoordinated, asynchronous change of individual values. Best used when
the value will undergo frequent, complex updates. Suitable when the result
of an update does not need to be known immediately.

Atom: Shared, mutable variable providing independent,
uncoordinated, synchronous change of individual values. Best used when
the value will undergo frequent reads but only occasional, though complex,
updates. Suitable when the result of an update must be known immediately.

AtomicReference: A simple object reference that can be
atomically. Updates are synchronous but fast. Best used when updates a
simple set operations. Not suitable when updates are complex.
AtomicBoolean and AtomicFixnum are similar
but optimized for the given data type.

Exchanger: Shared, stateless synchronization point. Used
when two or more threads need to exchange data. The threads will pair then
block on each other until the exchange is complete.

MVar: Shared synchronization point. Used when one thread
must give a value to another, which must take the value. The threads will
block on each other until the exchange is complete.

ThreadLocalVar: Shared, mutable, isolated variable which
holds a different value for each thread which has access. Often used as
an instance variable in objects which must maintain different state
for different threads.

TVar: Shared, mutable variables which provide
coordinated, synchronous, change of many stated. Used when multiple
value must change together, in an all-or-nothing transaction.

Constructor Details

The :validator option must be nil or a side-effect free proc/lambda
which takes one argument. On any intended value change the validator, if
provided, will be called. If the new value is invalid the validator should
return false or raise an error.

The :error_handler option must be nil or a proc/lambda which takes two
arguments. When an action raises an error or validation fails, either by
returning false or raising an error, the error handler will be called. The
arguments to the error handler will be a reference to the agent itself and
the error object which was raised.

The :error_mode may be either :continue (the default if an error
handler is given) or :fail (the default if error handler nil or not
given).

If an action being run by the agent throws an error or doesn't pass
validation the error handler, if present, will be called. After the
handler executes if the error mode is :continue the Agent will continue
as if neither the action that caused the error nor the error itself ever
happened.

If the mode is :fail the Agent will become #failed? and will stop
accepting new action dispatches. Any previously queued actions will be
held until #restart is called. The #value method will still work,
returning the value of the Agent before the error.

Instance Attribute Details

#error_mode ⇒ undocumented(readonly)

The error mode this Agent is operating in. See #initialize for details.

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# File 'lib/concurrent/agent.rb', line 183deferror_mode@error_modeend

Class Method Details

.await(*agents) ⇒ Boolean

Blocks the current thread (indefinitely!) until all actions dispatched
thus far to all the given Agents, from this thread or nested by the
given Agents, have occurred. Will block when any of the agents are
failed. Will never return if a failed Agent is restart with
:clear_actions true.

NOTE Never, under any circumstances, call any of the "await" methods
(#await, #await_for, #await_for!, and #wait) from within an action
block/proc/lambda. The call will block the Agent and will always fail.
Calling either #await or #wait (with a timeout of nil) will
hopelessly deadlock the Agent with no possibility of recovery.

.await_for(timeout, *agents) ⇒ Boolean

Blocks the current thread until all actions dispatched thus far to all
the given Agents, from this thread or nested by the given Agents, have
occurred, or the timeout (in seconds) has elapsed.

NOTE Never, under any circumstances, call any of the "await" methods
(#await, #await_for, #await_for!, and #wait) from within an action
block/proc/lambda. The call will block the Agent and will always fail.
Calling either #await or #wait (with a timeout of nil) will
hopelessly deadlock the Agent with no possibility of recovery.

.await_for!(timeout, *agents) ⇒ Boolean

Blocks the current thread until all actions dispatched thus far to all
the given Agents, from this thread or nested by the given Agents, have
occurred, or the timeout (in seconds) has elapsed.

NOTE Never, under any circumstances, call any of the "await" methods
(#await, #await_for, #await_for!, and #wait) from within an action
block/proc/lambda. The call will block the Agent and will always fail.
Calling either #await or #wait (with a timeout of nil) will
hopelessly deadlock the Agent with no possibility of recovery.

Instance Method Details

Dispatches an action to the Agent and returns immediately. Subsequently,
in a thread from a thread pool, the #value will be set to the return
value of the action. Appropriate for actions that may block on IO.

#await ⇒ Boolean

Blocks the current thread (indefinitely!) until all actions dispatched
thus far, from this thread or nested by the Agent, have occurred. Will
block when #failed?. Will never return if a failed Agent is #restart
with :clear_actions true.

Returns a reference to self to support method chaining:

current_value=agent.await.value

NOTE Never, under any circumstances, call any of the "await" methods
(#await, #await_for, #await_for!, and #wait) from within an action
block/proc/lambda. The call will block the Agent and will always fail.
Calling either #await or #wait (with a timeout of nil) will
hopelessly deadlock the Agent with no possibility of recovery.

Returns:

(Boolean)
—

self

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# File 'lib/concurrent/agent.rb', line 349defawaitwait(nil)selfend

#await_for(timeout) ⇒ Boolean

Blocks the current thread until all actions dispatched thus far, from this
thread or nested by the Agent, have occurred, or the timeout (in seconds)
has elapsed.

NOTE Never, under any circumstances, call any of the "await" methods
(#await, #await_for, #await_for!, and #wait) from within an action
block/proc/lambda. The call will block the Agent and will always fail.
Calling either #await or #wait (with a timeout of nil) will
hopelessly deadlock the Agent with no possibility of recovery.

#await_for!(timeout) ⇒ Boolean

Blocks the current thread until all actions dispatched thus far, from this
thread or nested by the Agent, have occurred, or the timeout (in seconds)
has elapsed.

NOTE Never, under any circumstances, call any of the "await" methods
(#await, #await_for, #await_for!, and #wait) from within an action
block/proc/lambda. The call will block the Agent and will always fail.
Calling either #await or #wait (with a timeout of nil) will
hopelessly deadlock the Agent with no possibility of recovery.

#failed? ⇒ BooleanAlso known as:
stopped?

#restart(new_value, opts = {}) ⇒ Boolean

When an Agent is #failed?, changes the Agent #value to new_value
then un-fails the Agent so that action dispatches are allowed again. If
the :clear_actions option is give and true, any actions queued on the
Agent that were being held while it was failed will be discarded,
otherwise those held actions will proceed. The new_value must pass the
validator if any, or restart will raise an exception and the Agent will
remain failed with its old #value and #error. Observers, if any, will
not be notified of the new state.

true if all enqueued but unprocessed
actions should be discarded on restart, else false (default: false)

Returns:

(Boolean)
—

true

Raises:

(Concurrent:AgentError)
—

when not failed

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# File 'lib/concurrent/agent.rb', line 423defrestart(new_value,opts={})clear_actions=opts.fetch(:clear_actions,false)synchronizedoraiseError.new('agent is not failed')unlessfailed?raiseValidationErrorunlessns_validate(new_value)@current.value=new_value@error.value=nil@queue.clearifclear_actionsns_post_next_jobunless@queue.empty?endtrueend

#send(*args, &action) {|agent, value, *args| ... } ⇒ Boolean

Dispatches an action to the Agent and returns immediately. Subsequently,
in a thread from a thread pool, the #value will be set to the return
value of the action. Action dispatches are only allowed when the Agent
is not #failed?.

The action must be a block/proc/lambda which takes 1 or more arguments.
The first argument is the current #value of the Agent. Any arguments
passed to the send method via the args parameter will be passed to the
action as the remaining arguments. The action must return the new value
of the Agent.

#send!(*args, &action) {|agent, value, *args| ... } ⇒ Boolean

Dispatches an action to the Agent and returns immediately. Subsequently,
in a thread from a thread pool, the #value will be set to the return
value of the action. Action dispatches are only allowed when the Agent
is not #failed?.

The action must be a block/proc/lambda which takes 1 or more arguments.
The first argument is the current #value of the Agent. Any arguments
passed to the send method via the args parameter will be passed to the
action as the remaining arguments. The action must return the new value
of the Agent.

Dispatches an action to the Agent and returns immediately. Subsequently,
in a thread from a thread pool, the #value will be set to the return
value of the action. Action dispatches are only allowed when the Agent
is not #failed?.

The action must be a block/proc/lambda which takes 1 or more arguments.
The first argument is the current #value of the Agent. Any arguments
passed to the send method via the args parameter will be passed to the
action as the remaining arguments. The action must return the new value
of the Agent.

#send_off!(*args, &action) {|agent, value, *args| ... } ⇒ Boolean

Dispatches an action to the Agent and returns immediately. Subsequently,
in a thread from a thread pool, the #value will be set to the return
value of the action. Action dispatches are only allowed when the Agent
is not #failed?.

The action must be a block/proc/lambda which takes 1 or more arguments.
The first argument is the current #value of the Agent. Any arguments
passed to the send method via the args parameter will be passed to the
action as the remaining arguments. The action must return the new value
of the Agent.

Dispatches an action to the Agent and returns immediately. Subsequently,
in a thread from a thread pool, the #value will be set to the return
value of the action. Action dispatches are only allowed when the Agent
is not #failed?.

The action must be a block/proc/lambda which takes 1 or more arguments.
The first argument is the current #value of the Agent. Any arguments
passed to the send method via the args parameter will be passed to the
action as the remaining arguments. The action must return the new value
of the Agent.

Dispatches an action to the Agent and returns immediately. Subsequently,
in a thread from a thread pool, the #value will be set to the return
value of the action. Action dispatches are only allowed when the Agent
is not #failed?.

The action must be a block/proc/lambda which takes 1 or more arguments.
The first argument is the current #value of the Agent. Any arguments
passed to the send method via the args parameter will be passed to the
action as the remaining arguments. The action must return the new value
of the Agent.

#wait(timeout = nil) ⇒ Boolean

Blocks the current thread until all actions dispatched thus far, from this
thread or nested by the Agent, have occurred, or the timeout (in seconds)
has elapsed. Will block indefinitely when timeout is nil or not given.

Provided mainly for consistency with other classes in this library. Prefer
the various await methods instead.

NOTE Never, under any circumstances, call any of the "await" methods
(#await, #await_for, #await_for!, and #wait) from within an action
block/proc/lambda. The call will block the Agent and will always fail.
Calling either #await or #wait (with a timeout of nil) will
hopelessly deadlock the Agent with no possibility of recovery.